1. Field of the Invention
The present invention relates to the preparation of a mass spectrometer sample from the effluent of a gas chromatograph.
2. Description of the Prior Art
Gas chromatography is very useful as a separator of compounds, while a mass spectrometer is an excellent instrument for identifications. An effective interface would allow each instrument to operate in sequence, without degrading the performance of either. By separating the components of a specimen in a gas chromatograph, more accurate determinations with a mass spectrometer appear possible. However, interfacing a gas chromatograph with a mass spectrometer presents two problems: (1) separation of the sample from the chromatograph carrier gas; and (2) the time between the different chromatograph peaks as they reach the mass spectrometer as compared to the necessary or desired scanning time of the spectrometer.
The first problem noted above arises from the fact that gas chromatographs normally operate at pressures greater than 760 torr, while mass spectrometers function best at high vacuums in the 10.sup.-5 torr range, or greater. Fourier Transform mass spectrometers operate optimally near 10.sup.-8 torr. Thus, an interface between a gas chromatograph and a mass spectrometer has required a "throwing away" of some or most of the sample, with the resultant loss of sample size sensitivity. This problem is even more pronounced with packed column gas chromatographs due to the higher quantity of carrier gas in the column which must be removed before the sample can be introduced into the mass spectrometer.
The customary approach for overcoming the problems due to the pressure differences between the two instruments is to separate the carrier gas from the sample. Conventional methods of separating the carrier gas include: effluent splitting or Watson Biemann separators; jet separators; and molecular separating membranes. These methods provide differing degrees of sample enrichment, but none provide 100% sample transmission.
Watson Biemann separators are based on the concept of enrichment by diffusion. The lighter carrier gas molecules permeate an effusion barrier, such as sintered glass, in preference to the heavier organic sample molecules and can be removed by a vacuum system. Although the separation procedure does increase the sample-to-carrier gas ratio approximately 50 times, less than 50% of the sample passes into the mass spectrometer, resulting in a decrease in sensitivity due to the smaller sample size.
A precisely aligned, supersonic jet/orifice system may also be used to remove the carrier gas using the effusion principle. As the gas chromatograph effluent passes through a small jet, the stream is directed toward an orifice. The concentration of carrier gas increases away from the center line while the concentration of the sample tends to increase toward the center. The orifice intercepts only the center portion of the stream. Two such jet/orifice assemblies may be used in series if desired. Using this method, approximately 60% of the sample is transmitted to the mass spectrometer with a sample enrichment of approximately 100.
Molecular membrane separators take advantage of differences in the permeability rate of the sample and the carrier gas through a silicone rubber membrane. The column effluent from the gas chromatograph passes a thin rubber membrane. The carrier gas usually has a low permeability and is not adsorbed by the membrane, whereas the organic molecules are adsorbed and pass through the membrane and directly into the high vacuum of the mass spectrometer. Sample transmission rates vary between 50 and 90 percent, and the enrichment factor is approximately 1000.
Another technique is disclosed in U.S. Pat. No. 3,896,661 to Parkhurst et al which proposes the use of thin layer chromatography of the mixture to be analyzed. The organic portion of the effluent of a gas chromatograph is placed on a chromatographic medium. The sample components migrate at different rates on the medium. The medium is then selectively heated to sublime the adsorbed chemical substance directly into the ion source of the mass spectrometer. U.S. Pat. No. 4,267,457 to Nakagawa et al shows a similar system in which a sample holding element is composed of a porous and gas permeable aggregate of ingredients which allow the components of a sample to separate to form a chromatogram.
The systems of the above noted patents do not use a gas chromatograph and thus do not deal with the interface between a gas chromatograph and a mass spectrometer. Indeed, the separation of the sample components does not utilize the precise separation capabilities of a gas chromatograph in connection with a mass spectrometer to enhance the analysis of the sample.
The second noted problem which arises when interfacing gas chromatographs to mass spectrometers is the small time interval that may exist between two or more components in the gas chromatograph effluent. If the time required for scanning over the mass range of interest is longer than the time between the chromatograph peaks, the resulting mass spectrum is a mixture of the components. In the case of high resolution, capillary column gas chromatographs, the time between peaks can be less than a second. This has limited the gas chromatograph/mass spectrometer performance for all types of mass spectrometers and made it impossible to take full advantage of the capabilities of the high resolution mass spectrometers when interfaced with a gas chromatograph.
Although the Fourier transform mass spectrometer is a fast scanning instrument (up to 100 scans/second), the use of this speed in capillary gas chromatograph interfacing is not practical for several reasons. First, mass resolution obtained under such fast scanning rates is poor. Second, the signal-to-noise ratio is low due to the lack of time for adequate signal averaging of each gas chromatograph peak. To achieve high resolution mass spectra, detection time in the order of one second is needed. For a good signal-to-noise ratio, signal averaging for a few seconds is desirable. A third, but less important, problem with this method is the lack of time for Fourier transformation and the necessity of a large storage module in which to "dump" all of the raw data for later transformation.